NbTi alloy has long served as the backbone of the superconducting wire industry. Despite continued research into alternate materials, including the recent interest in high temperature superconductors, NbTi remains the superconductor of choice. Unfortunately, high quality NbTi conductors are difficult to produce, expensive, and requiring carefully controlled alloy melting operations and complicated thermo-mechanical work schedules. Methods for simplifying these operations without adversely affecting critical superconducting properties would be of great value to the superconducting wire industry.
The conventional method for the fabrication of NbTi conductors begins with the melting of the alloy. Generally, an electrode is fabricated from pure niobium and pure titanium and is then electron beam melted. The resulting ingot is vacuum arc melted at least three times to ensure high homogeneity (.+-.1.5 wt % composition). This is necessary because inhomogeneous material results in poor conductor ductility and/or inferior current density (J.sub.c).
The NbTi ingot is inserted into an extrusion can, which is isolated from the NbTi by a suitable diffusion barrier. Typically, the extrusion can is made of copper and the barrier is made of niobium. The barrier is required to prevent the formation of brittle Cu-Ti intermetallics during subsequent processing. Such intermetallics can cause severe mechanical problems when the later multifilament wire is being reduced to final size.
The monofilament NbTi billet is welded shut and then extruded into a rod. This material is drawn to wire, straightened, cut, and restacked into another copper extrusion can. This multifilament billet is welded shut and then hot isostatically pressed (HIP'd) to collapse the void space. Next, the billet is extruded into a rod. At this point, a series of heat treatment and cold drawing steps begins. These steps are necessary in order to obtain useful current densities in the final NbTi conductor.
As is well known in the art, high current densities are achieved only when an adequate defect structure is present in the NbTi. The defects serve to inhibit the motion of magnetic flux lines and thereby help prevent the superconductor from going normal, i.e., non-superconducting --when supporting large transport supercurrents. The defects may take any of a number of forms: grain boundaries, different phases, dislocations, or impurities. By far the most effective pinning defect in NbTi is a finely distributed secondary phase which is normal at the temperatures and magnetic fields of interest. The introduction of such a phase is the purpose behind the heat treatment of NbTi alloys.
After extrusion, the NbTi filaments in a conductor are essentially homogeneous single phase .beta. NbTi. Only the weaker defects described above are present. The benefits of normal phase pinning can only be obtained by taking advantage of the precipitation of .alpha. Ti from the .beta. NbTi alloy. As shown in the NbTi phase diagram of FIG. 1, at temperatures below 882.degree. C., a two phase equilibrium state exists between .alpha. Ti and .beta. Nb. A Nb-55 wt. % Ti alloy, for example, when brought to equilibrium at 400.degree. C., dissociates by eutectoid decomposition; NbTi.fwdarw..alpha. Ti+.beta. Nb, into nearly pure .alpha. Ti (about 96 wt % Ti) and niobium rich .beta. Nb phase (Nb-45% Ti) in a volume ratio of about 1:4. These phases have completely different crystallographic structures. The .alpha. Ti has a hexagonal close pack (HCP) structure while the .beta. Nb has a body center cubic (BCC) structure. Since the .alpha. Ti is nearly pure titanium, it can serve as a normal defect. Insofar as the initial alloy is homogeneous, the .alpha. Ti is distributed throughout the NbTi, usually precipitating at .beta. NbTi grain boundaries. For optimum pinning, the thicknesses and spacings of the .alpha. Ti zones must be on the order of the superconductor coherence length (about 50 A.degree.) which can only be achieved by a substantial wire reduction after precipitation.
In practice, more than one precipitation heat treatment is required to produce optimum results in the NbTi multifilament. A typical thermo-mechanical schedule for the commonly used Nb 46.5 wt % Ti will involve three or more 300.degree. C.-450.degree. C. heat treatments, 40-80 hours in duration, separated by areal reductions of approximately 2.6. The final areal reduction, which reduces the accumulated .alpha. Ti to its optimum size and spacing, is usually in the range of 50-100. The best of the thermo-mechanical schedules for Nb 46.5 wt % Ti produces about 20 volume percent of .alpha. Ti in the NbTi and J.sub.c 's in excess of 3000 A/mm.sup.2 at 5T and 4.2K. In wires with these properties, the .alpha. Ti is configured in a dense array of ribbons 10-20 A.degree. in thickness, 40-80 A.degree. apart, and with an aspect ratio dependent upon the final strain imparted (see, for example, "Restricted Novel Heat Treatments for Obtaining High J.sub.c in Nb 46.5 wt % Ti", P. J. Lee, J. C. McKinnell, and D. C. Larbalestier, Advances in Cryogenic Engineering (Materials), vol. 36A, pp. 387-294, Ed. R. P. Reed and F. R. Fickett, Plenum Press, New York, 1990).
While it is necessary to apply heat treatments in order to precipitate .alpha. Ti and obtain high current density, these heat treatments also create mechanical problems. The presence of .alpha. Ti greatly reduces the ductility of the NbTi. At the same time, the heat treatments result in titanium diffusion into the niobium barrier around the NbTi and out to the copper matrix. Brittle Cu-Ti intermetallics can then form. Both the reduced ductility and the Cu-Ti formation cause filament deformation, called "sausaging", wire breakage, and degraded superconductor performance. The diffusion through the barrier also causes titanium depletion in the NbTi, contributing to depressed superconducting properties.
The present invention eliminates these problems and the need for any precipitation heat treatments. In essence, the present invention is the exact opposite of the conventional NbTi processing route. Instead of fabricating the NbTi alloy and then precipitating the necessary amount of nearly pure titanium the present invention begins with pure niobium and pure titanium and then combines the two to form two metallurgically separate and distinct phases which can co-exist in equilibrium at a given temperature. Only a brief equilibrium heat treatment may then be required to fix the .alpha. Ti and .beta. Nb phases and to create a structure identical to that of a conventionally processed alloy.
In the preferred embodiment of the invention pure niobium and pure titanium sheets having a thickness ratio of about 1:2 are layered alternately within a ductile extrusion can. This composite is then reduced via standard extrusion and wire fabrication techniques to a size at which each pair of Nb+Ti is about 2 .mu.m in thickness. In the course of bringing the material to this size, sufficient temperature and times are employed to cause the niobium and titanium to alloy. At the 2 .mu.m layer thickness, a separate heat treatment may be applied in order to bring the partially diffused structure to a uniform .alpha. Ti and .beta. Nb equilibrium concentration at the specified temperature. After the heat treatment, the composite is reduced in size to the point where each Nb+Ti pair is about 100 nm in thickness, so that optimum flux pinning is obtained. The fine structure within this product is indistinguishable from a comparable alloy that has been subjected to multiple heat treatments.
Because the process of the present invention does not involve the extensive precipitation heat treatments of the conventional fabrication route, titanium diffusion through filament barriers does not occur. Sausaging and depressed superconducting properties are therefore not a problem. Although one might expect that the presence of pure titanium might cause ductility problems, much as .alpha. Ti does in the conventional alloy, no such problems arise so long as the interstitial content of the pure titanium is not too high. In particular, if the oxygen content is kept below about 1000 ppm, composite ductility is excellent. It is believed that this is due to the fact that the titanium is in all cases adjacent to niobium. Since processing is performed at high temperature, the niobium, with less than 100 ppm of oxygen, is able to getter much of the oxygen from the titanium, thereby improving its deformability. By eliminating poor ductility, filament sausaging, and titanium depletion, the present invention greatly improves upon the current methods for the fabrication of fine filament NbTi superconductors.
Other researchers, intent upon obtaining improved superconductor performance, have developed composites superficially similar to those of the present invention. In U.S. Pat. No. 3,625,662, Roberts et al. describe a method for fabricating superconductors through the diffusion of pure metal layers. The product of Roberts '662 is entirely different from that of the present invention in that the claimed improvement is obtained by maximizing the surface-to-volume ratio of the superconductor, not by creating a structure which is the same as that of a heat treated alloy. In fact, in FIG. 2a of Robert's '662 patent he shows a distinct region of unreacted pure niobium and pure zirconium remaining after diffusion has taken place. He also indicates that Nb-Zr and Nb-Ti are solid solution alloys and that for Nb-Zr, diffusion should take place at temperature as high as 1800.degree. F. (982.degree. C.). The allotropic .alpha. to .beta. phase transformation for pure zirconium occurs at 862.degree. C. The temperature of 1800.degree. F. would be well above the two phase region and separate phases can not exist in this region. Roberts '662 in no way recognized the need for pinning regions and does not mention the two phase structure at all that is central to the present invention.
In U.S. Pat. No. 4,803,310, Zeitlin et al. describe a method by which to fabricate a superconducting composite consisting of superconducting alloy filaments embedded in a continuous, non-random network of normal pinning material. The normal layers are designated so that when the material is brought to the proper size, the layers have a thickness and spacing that match the flux line lattice (FLL) at some predetermined temperature and applied magnetic field. Maximum flux pinning is claimed to result. Although the result of the process is a two-phase structure, Zeitlin '310 teaches that the normal component should be "one that will not diffuse, or will diffuse only nominally into the core filaments" (column 2, lines 47-48). The inventors thus ensure the two-phase structure by preventing diffusion. Indeed, for the example of manufacture, any significant interdiffusion between the niobium and NbTi would be expected to degrade the superconducting properties by shifting the NbTi composition to higher niobium content and by undermining the two-phase structure. This effect has been confirmed experimentally ("Further Developments in NbTi Superconductors with Artificial Pinning Centers", H. C. Kanithi, P. Valaris, L. R. Motowidlo, and B. A. Zeitlin, To Be Published, Presented as paper no. DX-4(C), ICMC/CEC, Huntsville, Ala., June 1991). In contrast, the present invention requires diffusion to produce the .alpha.+.beta. structure.